Page "Ionosphere" Paragraph 21

When the Sun is active, strong solar flares can occur that will hit the sunlit side of Earth with hard X-rays.

The X-rays will penetrate to the D-region, releasing electrons that will rapidly increase absorption, causing a High Frequency ( 3-30 MHz ) radio blackout.

During this time Very Low Frequency ( 3 – 30 kHz ) signals will be reflected by the D layer instead of the E layer, where the increased atmospheric density will usually increase the absorption of the wave and thus dampen it.

As soon as the X-rays end, the sudden ionospheric disturbance ( SID ) or radio black-out ends as the electrons in the D-region recombine rapidly and signal strengths return to normal.